docs: def _archive(self):
          """
          Performs an archive action on the todolist.
  
          This means that all completed tasks are moved to the archive file
          (defaults to done.txt).
          """
          archive, archive_file = _retrieve_archive()
  
          if self.backup:
              self.backup.add_archive(archive)
  
          if archive:
              from topydo.commands.ArchiveCommand import ArchiveCommand
              command = ArchiveCommand(self.todolist, archive)
              command.execute()
  
              if archive.dirty:
                  archive_file.write(archive.print_todos())